The treatment of cultured animal cells with hypertonic growth media to reversibly inhibit polypeptide chain initiation has gained widespread acceptance as a useful tool for the unmasking of virus specific polypeptide synthesis in virus infected cells; for the construction of translation maps of viral genomes; for comparing, in vivo, the relative intrinsic translational efficiencies of individual mRNAs; and in studies aimed at understanding translational control mechanisms. This method of inhibiting in vivo translation has several major advantages over other existing methods: It is selective for inibition of the initiation step while elongation, termination, and processing of polypeptides proceed normally; inhibition is immediately reversible permitting synchronous reinitiation only at correct initiation sites; and this treatment results in a differential inhibition in the synthesis of virus specific, specialized and other cellular polypeptides. However, the usefulness of this technique is somewhat limited, at present, by the fact that essentially nothing is known about the molecular basis of hypertonic treatment-mediated inhibition of protein synthesis. We propose to use available methods to ask the following questions about the molecular consequences of hypertonic treatment of virus-infected and uninfected animal cells as they relate to the translati nal process. At what step in the process of polypeptide chain initiation does the lesion occur? What are the effects of hypertonic treatment on the activity and physical state of the translational machinery? Is it possible to reproduce in vitro the differential inhibition in the synthesis of individual viral and cellular polypeptides which has been observed in vivo after hypertonic treatment? Answers to these questions will provide us with an increased understanding of the molecular basis for the difference in relative translational efficiencies of individual mRNAs and the role of ion concentrations in the regulation of the translation process in virus infected and uninfected animal cells.